1. Field of the Invention
The present invention relates to a thermal image transfer recording medium, which is free from sticking phenomenon and blocking phenomenon, and has excellent preservability and printing performance.
2. Discussion of Background
In a conventional thermal image transfer recording system, an image receiving sheet is superimposed on a thermal image transfer recording medium which comprises a support and a thermal image transfer ink layer formed thereon in such a manner that the image receiving sheet is brought into pressure contact with the thermal transfer ink layer, a thermal head is brought into contact with the back side of the support which is opposite to the thermal image transfer ink layer, and the thermal head is heated with the application of a pulse-shaped signal current thereto, whereby the thermal image transfer ink layer is imagewise fused or sublimated to form images on the image receiving sheet.
In a thermal fusing image transfer system, in particular when black color printing is performed, a thermal image transfer recording medium with an ink layer which contains carbon black is employed.
Furthermore, in the case of color printing, three separate thermal transfer recording media with primaries, yellow, magenta and cyan, are prepared and colored images are printed by successively superimposing these thermal transfer recording media on an image receiving sheet.
In a thermal sublimation image transfer system, the amount of a sublimable ink to be sublimated is controlled by the amount of thermal energy applied thereto by a thermal head, so that image density gradation printing can easily be performed. Specifically, images with high density can be printed by increasing the voltage to be applied to the thermal head or the length of the application time of the current pulses applied thereto, and images with low density are printed by decreasing the voltage to be applied to the thermal head or the length of the application time of current pulses. Color printing can be performed in the same manner as by the thermal fusing image transfer system, and color printing with image density gradation can be performed by changing the amount of thermal energy applied by the thermal head.
As the material for the support of a thermal fusing image transfer medium and a sublimation type thermal image transfer medium, plastics films such as a polyethylene terephthalate (PET) film are in general use. However, when plastics films such as a PET film are employed as the support, a portion of the film with which a thermal head comes into contact is fused by the heat applied thereto through the thermal head, and is stuck on the thermal head during an image transfer step. The result is that the recording medium cannot be transferred at a stable speed. When this sticking phenomenon takes place, not only the printing quality is considerably degraded, but also the fused film makes a big noise when it is peeled away from the thermal head, or the transportation of the recording medium is stopped by the thermal head to which the fused film adheres, or the recording medium is broken by the thermal head.
In order to prevent this sticking phenomenon, there have been proposed various methods of providing a heat resistant protective layer on one side of the support of the recording medium with which a thermal head comes into contact, that is, on the side opposite to the thermal image transfer ink layer.
For example, Japanese Laid-Open Patent Applications 55-7467 and 63-172688 propose the provision of a heat resistant protective layer comprising silicone resin, epoxy resin, melamine resin, phenolic resin, fluorine plastics, polyimide resin, polyamide resin, or cellulose resin, on one side of the support.
Furthermore, Japanese Laid-Open Patent Application 60-201989 proposes the provision of a heat resistant protective layer comprising an aromatic polyamide; and Japanese Laid-Open Patent Application 63-203387 proposes the provision of a heat resistant protective layer comprising an aromatic polysulfone resin.
These heat resistant protective layers, however, cannot prevent the occurrence of the sticking phenomenon sufficiently for use in practice, and when a thermal image transfer recording medium with such a heat resistant protective layer is preserved in the form of a roll, the heat resistant protective layer and the thermal image transfer ink layer of the thermal image transfer recording medium stick together, that is, the so-called blocking phenomenon takes place.
When such a blocking phenomenon takes place, the thermal image transfer recording medium in the form of a roll cannot be transferred smoothly, and ink components from the ink layer are transferred to the heat resistant protective layer and fixed thereon, and are then transferred to, deposited on, or fixed on a heating element portion of a thermal head and its vicinity, so that printing quality obtained is significantly degraded.
Japanese Laid-Open Patent Application 61-143195 proposes the provision of a back coat layer (i.e. a heat resistant protective layer) which comprises a silicone graft copolymer or a block acrylic copolymer. The back coat layer comprises a silicone graft copolymer or a block acrylic copolymer and has excellent slip properties, and therefore an ant-sticking performance, and excellent heat resistant performance and anti-blocking performance. However, the silicone graft copolymer and block acrylic copolymer are poor in film formation properties, so that when continuous printing is performed, the surface portion of the back coat layer comprising such a copolymer is scraped therefrom by a thermal head, and scraped components from the back coat layer are fused and adhere to a heating element of the thermal head and its vicinity. As a result, the printing quality obtained is significantly degraded in the same manner as when the blocking phenomenon takes place. In order to obtain excellent printing quality, the thermal head must be cleaned very frequently, but this is troublesome.
Furthermore, Japanese Laid-Open Application 1-221281 proposes a thermal image transfer recording medium with the provision of a heat resistant protective layer comprising an ethyl cellulose resin with the addition of silicone oil and/or additives such as a surfactant on an ink layer. This method, however, has the shortcoming that the silicone oil and/or additives such as a surfactant are transferred to the ink layer while the recording medium is allowed to stand at high temperatures or preserved for a long period of time, and the transferred silicone oil and/or additives to the ink layer cause non-uniform image transfer and imperfect image transfer with the formation of non-printed portions, so that obtained printing quality is significantly degraded. This shortcoming is the same as that of a method of providing a resin layer which contains, for instance, a solid or semi-solid surfactant, which is disclosed in Japanese Laid-Open Patent Application 57-129789.
Japanese Laid-Open Patent Application 1-234292 discloses the provision of a heat resistant protective layer comprising an acetate-group-containing cellulose ester resin or this cellulose ester resin with the addition of silicone oil thereto. This protective layer, however, still has the same shortcomings as in the heat resistant protective layers disclosed in Japanese Laid-Open Patent Applications 55-7467 and 63-172688, and Japanese Laid-Open Patent Application 1-221281.
As mentioned above, varieties of a heat resistant protective layer for a thermal image transfer recording medium have been proposed, but most of them still have the shortcomings that transfer performance becomes poor, a thermal head is abraded, the support of the recording medium is broken, which degrades printing quality, when printing is carried out continuously for an extended period of time; and the anti-blocking performance and prevention of the formation of thermal head dust are insufficient for use in practice.